The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Toothaches are usually caused by injury or trauma to the tooth. Injury is commonly a result of dental decay (or cavity). A cavity is usually felt when it gets larger and deeper into the layers of tooth structure. The hard outer layer of tooth is called enamel, and the softer layer beneath enamel is called dentin.